1. Field
The various circuit and method embodiments described herein relate in general to current sensing circuits and techniques, and, more specifically, to circuits and techniques to synthesize a sense resistor using an active component to enable current sensing over a large dynamic range.
2. Background
Current sense resistors are widely known. An example of a typical current sense resistor circuit 10 and its use is shown in FIG. 1, in which a sense resistor 11, a control device, such as a MOSFET 12, and a load 14 are connected in series between a voltage supply 16 and a reference potential 18. The current in the sense resistor 11 develops an output voltage across the sense resistor 11, which can be sensed by a current sense comparator 20. Typically, the circuit 10 senses current using the resistor 11 and looks for a specific voltage via the current sense comparator 20, representing a current threshold. The current sense resistor circuit 10 is accurate, stable, and inexpensive, but requires a significant voltage across the sense resistor 11, so that offset and noise are insignificant.
The circuit 10 implements a linear regulator that controls load current based on a voltage input, but is weak, because as the control signal that is applied drops, the signal across the sense resistor 11 also drops, and the offset and noise of the comparator 20 limit accuracy and performance.
Thus, when the current sense threshold adjusted in a system in which a current sense resistor circuit is used, either a very accurate current sense comparator 20 with very low offset is required, or a high resistance sense resistor 11 needs to be used. A high resistance sense resistor, however, drops an excessive voltage at high current and a barely adequate voltage at low current. Therefore, this solution is inefficient because a lot of power is wasted at high current level, and serious demands are placed on the comparator 20 at low current level.
Another current sensing technique that has been used is a current mirror 30, as shown in FIG. 2. The current mirror 30 has a reference side having a reference current source 32 and MOSFET 34 connected in series and a mirror side having MOSFETs 36 and 12 and load 14 connected in series between the voltage source 16 and reference potential 18. The current from the reference current source 32 is driven through the MOSFET 34 to create a voltage on its gate and on the gate of a sensing MOSFET 36, thereby setting its resistance. The current through the sensing MOSFET 36 develops a voltage on its drain, which is sensed by the comparator 20.
Typically, the reference current portion of the current mirror 30 is provided from an external current. This allows scaling but does not actually generate a voltage proportional to current, which is used for feed-forward and other purposes.
What is needed is a circuit that provides a stable and predictable sense resistance that can sense control currents over a wide dynamic range to provide a constant voltage threshold for good accuracy and high efficiency.